The present invention generally relates to semiconductor processing, and in particular to a system and method for detecting wafer lines and contacts that are scummed or partially open in conjunction with a lithography process.
In the semiconductor industry, there is a continuing trend toward higher device densities. To achieve these high densities there has been and continues to be efforts toward scaling down the device dimensions (e.g., at submicron levels) on semiconductor wafers. In order to accomplish such high device packing density, smaller and smaller features sizes are required. This may include the width and spacing of interconnecting lines, spacing and diameter of contact holes, and the surface geometry such as comers and edges of various features.
The requirement of small features with close spacing between adjacent features requires high resolution photolithographic processes. In general, lithography refers to processes for pattern transfer between various media. It is a technique used for integrated circuit fabrication in which a silicon slice, the wafer, is coated uniformly with a radiation-sensitive film, the resist, and an exposing source (such as optical light, x-rays, etc.) illuminates selected areas of the surface through an intervening master template, the mask, for a particular pattern. The lithographic coating is generally a radiation-sensitive coating suitable for receiving a projected image of the subject pattern. Once the image is projected, it is indelibly formed in the coating. The projected image may be either a negative or a positive image of the subject pattern. Exposure of the coating through a photomask causes the image area to become either more or less soluble (depending on the coating) in a particular solvent developer. The more soluble areas are removed in the developing process to leave the pattern image in the coating as less soluble polymer.
Due to the extremely fine patterns which are exposed on the photoresist, Scanning Electron Microscopes (SEMs) are often employed to analyze and measure critical dimensions resulting from the lithographic process. Critical dimensions include the size of minimum features across the wafer such as linewidth, spacing, and contact dimensions. Although SEMs have been effective in providing quantitative measurement information related to critical dimensions of a wafer, they have not been as effective in providing qualitative analytical information regarding process related issues.
A particular process related issue is known as scumming and is generally related to the resolution of the image pattern, and may occur with either negative or positive resists. Scumming may result when a feature is underexposed, has poor focus, or is simply too small to print (e.g., mask defect or a subnominal test structure). As a result of the aforementioned resolution problems, line edges may not be well defined and scumming may occur between the lines or within a contact hole. This results in the areas between the lines and within the contact openings to only be partially open.
Conventional analytical CD-SEM systems for measuring critical dimensions of wafers often fail to detect lines and contact openings that may be scummed or only partially open. Thus, poorly processed wafers may provide adequate critical dimension measurements yet evade detection of scummed lines and contacts. For example, a scummed contact opening may be measured for the width of the contact opening. Even though the contact opening may only be partially open, conventional CD-SEM systems often indicate adequate width dimensions despite the fact that the contact opening may be defective.
It would therefore be desirable to have a system and/or method which substantially increases the probability that scummed lines and contacts will be detected during the measurement of critical dimensions.
The present invention is directed toward a system and method for detecting scummed lines and contacts in a film resulting from a lithographic process. A signal is provided by an SEM system during critical dimension measurements. By applying analytical signal processing techniques to the signal, a determination is made as to the quality of the underlying film undergoing the critical dimension measurement. By employing such techniques, various lithographic processes may be characterized effectively. This results in identifying a lithographic process which provides increased manufacturing yields and increased integrated circuit performance.
More particularly, the present invention performs a curve fitting analysis to the signal received from the SEM system during critical dimension measurements. It has been found that scummed lines and contacts produce a curved or generally nonlinear signal response over a portion of the received signal. In contrast, a generally flat signal response is provided by a properly manufactured wafer. Mathematical regression analysis provides a methodology for determining the amount of curvature or flatness in the measured signal whereby scummed lines and contacts may be detected by determining if a portion of the signal is above a predetermined threshold of curvature during the critical dimension measurement. It is noted that the invention disclosed herein may be applied to substantially any system that provides a shaped signal based on the scanning or measurement of a wafer surface.
To the accomplishment of the foregoing and related ends, the invention, then, comprises the features hereinafter fully described. The following description and the annexed drawings set forth in detail certain illustrative embodiments of the invention. These embodiments are indicative, however, of but a few of the various ways in which the principles of the invention may be employed. Other objects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the drawings.